In many radio systems, information is conveyed through phase or frequency modulation. In the past, phase or frequency modulation was formed by analog means such as applying the modulation signal to a varactor diode to warp the frequency versus time response of a voltage controlled oscillator. In more recent times, the phase or frequency modulation has been performed within the digital domain and applied to a fractional-N divider in the feedback loop of a fractional-N synthesizer. For example, see U.S. Pat. No. 6,834,084, entitled DIRECT DIGITAL POLAR MODULATOR, which issued Dec. 21, 2004 and is hereby incorporated herein by reference in its entirety. This technique has some significant advantages over the older analog techniques. Namely, the system has a digitally predictable and repeatable response, needs no calibration, and has lower cost and smaller size.
One difficulty encountered with digital phase or frequency modulation systems using a fractional-N frequency synthesizer is that the closed loop frequency response of a typical fractional-N synthesizer has a cut-off frequency that is not sufficient to pass many desired modulation formats within the required spectral and distortion limits. One solution to this problem is disclosed in U.S. Pat. No. 6,008,703, entitled DIGITAL COMPENSATION FOR WIDEBAND MODULATION OF A PHASE LOCKED LOOP FREQUENCY SYNTHESIZER, issued Dec. 28, 1999. In this system, the modulation fed to the fractional-N divider in the feedback loop of a fractional-N synthesizer is pre-distorted to compensate for the low-pass response of the fractional-N synthesizer.
However, there is a difficulty with the system disclosed in U.S. Pat. No. 6,008,703. Even with the pre-distortion in the feedback loop, the noise shaping of the fractional-N synthesizer limits the practical maximum bandwidth of the synthesizer to values that are marginal for systems such as the Enhanced Data Rate for Global Evolution (EDGE) 8-Level Phase Shift Keying (8PSK) modulation format and not sufficient for the 3G Wideband Code Division Multiple Access (WCDMA) modulation format. As such, there remains a need for an improved digital modulation system.